arm64: kernel: add code to set cpu boot mode to secondary_entry shim

The refactoring of el2_setup split code setting up EL2 and detecting the
CPU boot mode in separate chunks. This allows the code that sets up EL2 to
run in an endian independent way - ie before the endianess is set up in
the respective sctlr registers.

This patch brings secondary_entry up-to-date so that CPUs entering the
kernel through this code path set-up EL2 and the cpu boot mode properly.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Mark Rutland <mark.rutand@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>